One method of treating breast cancer is by radiotherapy. It may also be necessary to treat the parasternal lymph nodes as part of the course of breast cancer radiotherapy. Such radiotherapeutic treatment of the parasternal lymph nodes may lead to irradiation of large areas of normal tissue. This limits the total dose of radiation that can be delivered to the malignant tissue. Brachytherapy is a form of radiation therapy whereby a radiation source is placed in or near target malignant cells. One advantage of brachytherapy is the ability to deliver a higher dose of radiation while minimizing irradiation of normal tissue. Some methods of treating breast cancer, such as breast cancer metastases including metastases to the parasternal lymph nodes, include surgically cutting an internal thoracic artery and inserting a tube containing a source of radiation to treat parasternal lymph nodes after a mastectomy. This invasive procedure requires skilled surgeons, is mutilating and entails considerable risk for the patient, including post-operative infection. A minimally-invasive adjuvant method for treating breast cancer, such as breast cancer metastases, is needed. The present invention addresses this need.